Ferrous metal die casting process and products

ABSTRACT

A process for producing ferrous metal articles by pressure injection die casting at high temperature in molds lined with or having inserts of refractory metals of high thermal conductivity, particularly tungsten, molybdenum or their alloys, followed by short time heat treatments to produce desirable metallurgical structures. Due to the high rate of heat extraction and due to the turbulent stirring of the liquid metal by the injection process, even with the molds operating at temperatures elevated enough to prevent premature freezing and poor cast surfaces, the refractory metal of the mold produces a supercooling or quenching effect resulting in uniform, extremely fine as-cast grain structures which are unusually susceptible to short and more economical heat treatment to beneficially modify the structures and properties of the articles. Specific improved types of products of the processes of the invention having unusually fine and uniform grain structure, and greater strength and ductility than available in the prior art are cast irons and include: gray cast iron encased in malleable iron, malleable iron produced by heat treating die cast white iron for an unusually short period of time, and nodular or ductile iron.

United States Patent [72] Inventors Ronald L. Barto Wickliffe; Dallas T. Hurd, Gates Mills, both of Ohio 21 Appl. No. 718,640 [22] Filed Apr. 3, 1968 [45] Patented Oct. 26, 1971 [73] Assignee General Electric Company Continuation-impart of application Ser. No. 637,814, May 11, 1967, now Patent No. 3,532,561.

[54] FERROUS METAL DIE CASTING PROCESS AND PRODUCTS 16 Claims, 12 Drawing Figs. [52] US. Cl 148/3, 75/123,148/35,148/138,l64/113 [51] Int. Cl ..B22d 15/00, B22d 17/00, C2ld 5/00 [50] Field of Search 148/2, 3, 138,35; 29/180; 75/123; 164/1 13, 138 [56] References Cited UNITED STATES PATENTS 2,895,860 7/1959 Peras 148/3 2,906,651 9/1959 Saives. 148/3 2,906,653 9/1959 Peras 75/125 X FOREIGN PATENTS 516,696 1/1953 Belgium 516,698 l/1953 Belgium 554,894 2/1957 Belgium 1,183,362 1/1959 France 915,127 7/1964 U.S.S.R.

OTHER REFERENCES Morris et al., Die Casting lron and Steel, The lron Age pp. 1028-1030, 14, June 1933 Permanent Molds in Production of Crankshafts, pp. 665- 666, The lron Age, April 27, 1933 The lron Age, pp. 940- 943, June 1933 Materials and Methods, pp. 52- 54, Nov. 1949 Piwowarsky, Gusseisen 2nd Ed. 1951, See. h, pp. 130- 136 1.1. Goryunov, Precision Casting, Lenizdat pp. 49- 65, 1959 Unschau," Giesserei, pp. 280- 281, May 2, 1963 Von Horst Braun, Giesserei, p. 285- 292, April 29, 1965 Precision Metal Molding, pp. 54- 55, May 1960 Precision Metal Molding, p. 56 May 1960 Precision Metal Molding, pp. 28- 30, June 1962 Belov et a1., Russian Castings Production pp. 205- 207 May 9, 1964 Kudrin et 211., Russian Castings Production, pp. 31- 34 Jan. 1965 Voronin et al., Russian Castings Production, pp. 138- 140, March 1966 Precision Metal Molding, pp. 33- 36, April 1965 Precision Metal Molding, pp. 37- 38, Oct. 1965 Precision Metal Molding, pp. 31- 32, Feb. 1966 Metalworking News, pp. 22, May 9, 1966 Frommer/Lieby, Druckgiess-Technik," 2nd Ed. Vol. 1 p. 2 Springer-Verlag, Berlin, 1965 Precision Metal Molding, pp. 59- 61 April 1967 Modern Castings, pp. 87- 91 July 1967 Kobeler, Russian Castings Production, pp. 337- 340, July 1967.

Transactions of the American Foundrymens Society, Vol. 74,1966, pp. (66- 81) 32 1- 324 Primary Examiner-Charles N. Lovell Attorneys-Richard H. Burgess, Henry P. Truesdell, Melvin M. Goldenberg, Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg a supercooling or quenching effect resulting in uniform, extremely fine as-cast grain structures which are unusually cast white iron for an unusually short period of time, and nodular or ductile iron.

PAIENIEunm 26 I97! SHEET 3 UP 4 ig, 5b. GPRY IPON'HS C057- SHND CRS T DIE COST DIE 005T wmr: IRON-19$ ms 7'- 500x Fig 7b. MALLERBLE IEON-HNNERLED Inventors: RonaLd L. Bar-to DaLLas T. HUT'd y Mi/1 Their Adz't'orneg FERROUS METAL DIE CASTING PROCESS AND PRODUCTS This application is a continuation-in-part of our application Ser. No. 637,814, filed May ll, 1967, now US. Pat. No. 3,532,561.

BACKGROUND OF THE INVENTION This invention relates to processes for casting and heattreating graphitic ferrous metals, and to the products of such processes. More particularly, it relates to processes for producing precision parts or articles of commerce of ferrous metals by pressure injection die casting in relatively permanent molds, followed by certain heat treatments.

As is well known in the art, there are various disadvantages of economics and of product quality in each of the traditional methods of casting ferrous metals into useful objects. Sand casting, investment casting, and shell-mold casting each involve producing a new mold for each individual casting, thereby increasing the costs of the castings. Permanent mold processes for casting ferrous metals generally utilize relatively heavy insulating barriers between the molten metal and the mold itself. Thus, permanent mold techniques have in common with the other above-mentioned techniques the disadvantage of relatively slow cooling. Slow cooling of molten metal aggravates a tendency toward dendritic grain growth and inherent weaknesses in the cast product.

Pressure die casting processes originally developed for easting objects of aluminum, zinc, and other low melting metals cannot be used directly for casting ferrous metals because of inadequacies of the normal tool steel die materials for economical production of large numbers of castings repetitively in the same die. Reported experiments with use of refractory metal mold liners have permitted advances in ferrous metal pressure die casting, but such processes have not been fully developed for maximum technical and economic benefit, especially in combination with post-casting treatments. Ferrous liquid metal pressing in refractory metal molds can be useful when segregation in the casting can be tolerated, but the resulting segregated structures are not desirable for all purposes.

SUMMARY OF THE INVENTION Thus, it is an object of the present invention to provide a practical and economical process for repetitive pressure injection die casting of ferrous metals coupled with short heat treatments to produce castings with closely controlled dimensions, surface conditions, and metallurgical structures. Furthermore, an objective of the invention is to produce improved ferrous metal castings having greater strength, ductility, fine-grained structure for a substantial depth from the surface of the casting, and other properties generally superior to those obtainable from previously known casting methods, including an unusual ability to be beneficially altered or transformed in structure and properties by short and more economical post-casting thermal treatment.

Briefly stated, the present invention in one of its embodiments provides a method for pressure injection die casting of articles of ferrous metals containing at least 50 percent of iron and more carbon than the maximum amount that is soluble in the matrix phase, followed by short heat treatments. (Percentages herein are by weight except where indicated otherwise.) The casting is done in dies which are made of or have inserts or liners of certain refractory metals which have high heat transfer characteristics, adequate mechanical properties and high melting points. The refractory metals are efficiently thermally coupled to a heat sink, such as the casting machine itself together with the surrounding atmosphere or a cooling system, to permit rapid extraction of heat from the castings. Pressure injection into such molds results in effectively and rapidly supercooling the molten metal, yet the mold temperature can be high enough to prevent premature freezing that could cause surface defects in the castings. The resulting casting is ing at least 50 percent of either, or of combinations of tungsten and molybdenum, are provided over essentially all the interior surfaces of the die which come in contact with the molten ferrous metal being cast. ln order to obtain the preferred results, the refractory metal should have a thermal diffusivity of at least about l ft."/hr., a heat ditTusivity of at least about 40 B.t.u./ft. F. hr."*, and a melting point above about 3,000 F., and the layer is thick enough, depending on the thermal transfer characteristics of its backing, to enable rapid extraction of the heat of fusion from the molten metal and to mechanically withstand the rigors of high speed repetitive casting. The dies should be maintained at an elevated temperature such as about 500 F. or higher, depending on the particular ferrous alloy being cast, but below the freezing point of the ferrous metal being cast, while casting the metal, so as to essentially prevent surface irregularities and structural nonuniformities and discontinuities in the castings which would be caused by premature freezing of the liquid metal as it is being moved into the die. To the best of applicants knowledge, only the claimed refractory metals are capable of economic application in this process and demonstrate adequate heat transfer capabilities at the elevated temperatures required for successful ferrous metal pressure die casting, in combination with adequate ductility, strength and durability to withstand large number of repetitive die casting cycles.

Thermal diffusivity and heat diffusivity are useful measurements of the rate at which heat will be extracted from a casting by a mold material. Thermal diffusivity is defined as K/pC,,, where K is thermal conductivity in units of B.t.u./ft. F. hr., p is density in units of lb./ft. and C, is heat capacity in units of B.t.u./lb. F. Thermal diffusivity is a measure of how fast heat can be transferred through a mold. Heat diffusivity is defined as the square root of the product KpC and is a measure of the heat absorbing ability of a mold material. B.t.u. means British Thermal Units, lb. means pounds avoirdupois, ft. means feet, hr. means hour, and temperature is measured in degrees Fahrenheit.

The several specific embodiments of the invention include processes for producing articles of several types of alloys of iron and carbon, and also include the products of such processes. The alloys of articles produced according to the invention include gray iron encased in white iron which is converted to malleable iron on heat treatment, white which is converted to malleable iron on heat treatment, and nodular iron which is sometimes known as ductile iron and having the free carbon in the form of regular spheroidized nodules of graphite. Heat treatments of the invention alter the metallurgical structure in a way that can be seen by microscopy, improve at least certain mechanical properties, and have the effects of precipitating carbon from solution essentially all in the form of dispersed nodules in less than about 4 hours. The effects include specifically: for malleable iron, malleablizing the as-cast white iron and preferably producing a matrix grain size smaller than the irregular spheroids or particles of graphite in the structure; and for nodular iron, the development of spheroidal nodules and increasing the amount of graphite in the nodules.

The term essentially all" of the graphite being in the form of dispersed nodules means that the amount of graphite precipitated in other forms in the structure, except for the gray iron core of malleable-iron-encased articles, is not sumcient to significantly alter the properties of the metal. The term nodules includes both the irregular but rather compact spheroidal and nonflakey graphite typical of malleable iron. and also the regular spheroidal graphite particles typical of ductile iron; synonyms for ductile iron are nodular iron and spheroidal iron. These heat treatments can be much shorter or at lower temperatures than is necessary for similar effects in corresponding graphitic iron alloys cast in sand molds or by other conventional techniques, and, therefore, more economical. Continuing the heat treatment for a longer time will not avoid the essence of the present invention if the above-indicated results are obtained within the specified time periods; and it is well known in the art that, to some extent, equivalent results can be achieved with higher temperature and shorter time or lower temperature and longer time. The heat treatments of the invention can be performed in the conventional temperature ranges for corresponding sand-cast alloys, or at lower temperatures. In certain preferred embodiments of the invention, malleable iron with a gray iron core is produced with a heat treatment about in the range of l,500 to 1,900 F. for 10 minutes to 2 hours, malleable iron in the same ranges but also up to 4 hours, and ductile iron about in the range of 1,500 to 2,000 F. for 10 minutes to 2 hours.

The processes of the invention of pressure die casting followed by appropriate heat treatments result in great economy in that the heat treatments necessary to effect the desired metallurgical conversion require much less time, or lower temperatures, or both, than heat treatments for corresponding sand-cast metals and are more effective in that extremely fine grain structure and minimization of impurity segregation are retained in the final product. Moreover, heat treatments of the castings sometimes can utilize heat from'the casting process if the heat treatments are commenced before the newly formed castings have cooled completely.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic drawing of the central operating parts of a cold chamber pressure injection die casting machine, illustrating part of the method of the invention. In particular, it shows molten ferrous metal being poured into the chamber from which it is forced into the die for casting. The inner parts of the die are formed of massive refractory metal inserts.

FIG. 2 shows the same apparatus after the molten metal has been forced into the die and as it is solidifying.

FIG. 3 once again shows the same apparatus, but in this case after the die has opened to permit removal of the casting and its feeding system or sprue. FIG. 3a shows the die block with a different embodiment of the invention, that is, a relatively thin layer of refractory metal lining the die in place of the massive inserts of refractory metal of FIGS. 1 through 3.

FIG. 4 is the iron-rich end of the iron carbon phase diagram, locating specific compositions described herein.

FIGS. 5 through 8 are sets of photomicrographs showing the effects on metallurgical microstructure of the processes of the invention, in several cases as compared with prior art processes including sand casting and large ingot casting. In each figure, the microstructure for a particular pressure injection die cast ferrous metal alloy is shown as cast over the letter a and, where available, the corresponding microstructure produced by sand casting and equivalent in composition is shown over the letter b. Each of the photomicrographs is originally at a stated magnification before about one-third reduction for reproduction in the printed United States Patent. Therefore, the actual magnification of the figures as shown in the printed United States Patent will be about 67 percent of the stated magnification.

FIGS. 5a and 5b respectively show die cast and sand cast gray iron having a carbon equivalent of about 4.3 percent at a magnification of 200X.

FIG. 60 at 500x shows die cast white cast iron with a carbon equivalent of about 3.0 percent. Sand cast white cast iron would look about the same but with somewhat larger grain and particle sizes.

FIGS. 7a and 7b at 200x respectively show malleable iron produced by annealing die cast white cast iron of the type shown in FIG. 6a and comparable sand cast white iron. The malleable iron produced from die cast white iron was produced by heat treating or annealing 2 hours at l,650 F., a

greatly shorter time than is necessary for commercial production of malleable iron from sand cast white iron which normally is done for from 24 to 144 hours or more at l,650l ,850 F., even when chill plates are used to facilitate white iron production such as in thick sections. The die cast matrix grains are substantially smaller than the graphite nodules, which is not the usual malleable iron structure.

FIGS. 8a, 8b, and 8c at lOOX show unetched microstructures of nodular or ductile iron containing 3.6% percent carbon, 2.12% silicon, and trace amounts of other elements and impurities which have been inoculated while molten and just before casting with magnesium in amount of about 0.12 percent, added to the melt as ferrosilicon containing 5 percent magnesium. FIG. 8a shows the metal as die cast in accordance with the invention, but before heat treatment. Although small graphite particles are present in a white iron matrix, the typical ductile iron structure has not been fully developed. FIG. 8b shows the same metal as sand cast with a typical ductile structure. Essentially all free graphite is in the form of spheroidal nodules of regular shape and with rosette-structure at their centers. FIG. is the microstructure of the die cast ductile iron after a heat treatment of l,700 F. for 30 minutes to develop the ductile iron structure. By comparison with FIG. 8b, it is seen that the graphite nodule structure and grain size in the die cast metal of FIG. 8c is much finer, and it is well dispersed through the matrix. This structure gives superior mechanical properties.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In our efforts to develop and perfect the present invention, we have proven feasibility of the process and substantial improvements in product properties by pressure injection die casting and heat treating many parts of malleable iron with gray cast iron cores, parts of malleable iron, and parts of duc tile or nodular iron. It is expected that similar improvements could be obtained in other graphitic ferrous alloys.

The use of refractory metal die inserts maintained at elevated temperatures has been a central factor in achieving these successes. Table I below compares the thermal difiusivity and heat diffusivity of tungsten and molybdenum with a typical tool steel used for dies, AISI-SAE H-l I die steel, which has a composition of about 5.00% chromium, l.50% molybdenum, vanadium, 0.35% carbon, balance iron. The thermal diffusivity and heat diffusivity parameters were defined above in the Summary section.

TABLE I Heat Transfer Characteristics Various alloys of tungsten, molybdenum, or both together are also suitable as die inserts in accordance with the invention so long as the thermal diffusivity is greater than about I ft./hrs., the heat diffusivity is greater than about 40 Btu/ft. F. hr.", and the melting point is above about 3,000 F. Although the melting point of the refractory metal alloy itself should be above about 3,000 F., certain composite materials could be satisfactory having small amounts of lower melting materials dispersed through the refractory metal, such as copper-infiltrated tungsten, or composites of refractory metal particles or other shapes, such as wires, rods or plates, bonded together by minor amounts of lower melting metals, so long as there is not even incipient melting of separate minor phases at so low a temperature as to make the alloy or composite material unsuitable for reasons such as weakness. Also, thin coatings of lubricants, die washes, or other materials that do not interfere with characteristics of the invention can be used on the dies. The thermal diffusivity and heat difi'usivity depend on thermal conductivity, density and heat capacity and determine the rate at which castings can be produced in die inserts or composite cavities made in accordance with the present invention and the type of efi'ective solidification and cooling given to the castings. For practical application, it is desirable that the refractory metal die liners of the invention be repairable by building up material thickness by means such as welding or plasma spraying, or repairable by brazing, sintering, drilling and inserting plugs shrink fitted into holes, and otherwise.

Casting processes traditionally used for production of ferrous metal castings have involved a more or less protracted time for filling the mold with the molten metal, during which the liquid metal begins to freeze sequentially in zones before the mold filling process is completed. We have now discovered that it is possible, by the very rapid injection of molten ferrous metal under pressure into a die cavity within a heated mold comprising a material of relatively high thermal conductivity and having a melting point substantially higher than that of the metal being cast, to effect a very rapid and nearly uniform supercooling of the molten metal prior to its solidification so as to produce a precisely shaped, useful article of solid, cast metal. Such articles are characterized by smooth surfaces, an unusual degree of fineness of grain structure, and superior physical properties such as tensile and rupture strength, ductility, and, at least in some cases, corrosion resistance. A primary advantage of such cast articles is their increased susceptibility to various heat treatments which are more economical than those of the prior art and which retain and improve characteristics of the castings. Preferably, the entire mass of molten metal is injected into the heated die in a very short time, which can be less than about I second depending on the size of the mold. With most graphitic ferrous metal alloys being cast, solidification can be essentially completed in processes of the invention within less than about 2 seconds if the maximum section thickness of the casting is less than about one-half inch. For greater section thicknesses, very rapid solidification of the outer layers is still achieved, producing relatively thick, uniformly fine-grained, generally nondendritic surface layers, depending on the metal, from about one-fourth inch to one-half inch thick. Preferably, some turbulence of the liquid metal being injected under pressure through a restricted gate, followed by immediate application of the full design pressure of the die casting machine, permits the high heat transfer properties of the die lining to rapidly extract heat from the liquid metal.

In contrast with traditional pressure die casting practices where relatively high heat conductivity materials are cast in dies which have low heat conductivity, the present invention provides for the casting of metals of low heat conductivity in dies which have high heat conductivity. Therefore, beyond a certain point, the rate of heat transfer from casting to the mold during freezing will be determined primarily by the rate of transfer of heat energy from the interior of the casting through the frozen metal to the mold wall surface rather than the temperature differential at the interface between the mold and the casting. Also, relatively uniform, low temperature through the cross section of the molten metal leads to greater nucleation and finer as-cast grain size. The versatility of the present invention is further demonstrated by the realization that the ability to keep parts of the mold at quite high temperatures permits controlled uniform freezing of complex casting configurations to eliminate thermal stresses in the solidified metal which might otherwise be caused by nonuniform or zonal freezing and cooling. As is known in the art, heating or cooling methods can be applied in different sections of the mold to control different rates of heat removal from different parts of the castings, as desired.

To get smooth surfaces and uniform structures in the castings, the mold surfaces are kept at elevated temperatures, such as above about 500 F., or sometimes preferably in the range of about 700l,000 F., depending on the particular metal being cast, still maintaining a high degree of heat flow from the casting to and through the mold. This means that the use of thick coatings of carbonaceous or inorganic materials to insulate mold surfaces is not desirable for the purposes of the present invention; however, a thin coating or sooting to prevent adherence or soldering of the casting to the mold and to facilitate casting removal is permissible as long as it does not substantially restrict heat transfer.

As discussed above, heat treatments follow these casting operations to develop the desired metallurgical structures.

Turning now to the drawings, FIGS. 1 through 3 illustrate the'process of the invention. In FIG. 1, a conventional die casting machine, well known in the art, preferably of the coldchamber type, is illustrated at l by a box in dashed lines. A cold-chamber die casting machine can be distinguished from a hot-chamber machine in accordance with the following description. In a cold-chamber machine the molten metal 5 is transferred manually as by pouring from ladle 2 through metal supply opening 15, or automatically, to a shot-sleeve 3. Plunger 4 is designed to push the molten metal 5 from shotsleeve 3 through gate 16 into die cavity chamber 6 when plunger 4 is moved by an external power source as indicated at 7, such as a hydraulic cylinder. By contrast, a hot-chamber machine, which could be used with the present invention, provides for automatic pumping of the molten metal from beneath the surface of a holding tank of the metal, not shown in these drawings. Thus, the pressure source or pump in a hotchamber machine is normally immersed in the molten metal and operates at the temperature of the molten metal. It will be understood by those skilled in the art that a hot-chamber machine could be made to operate in the pressure die casting of ferrous metals if the metal pump was properly designed of suitable materials.

In the present invention, the die backup blocks comprise a movable half 8 and a fixed half 9. Massive inserts of refractory metals are illustrated at 10 and 11 and are fixed in each of the halves 8 and 9. Suitable means are provided for moving the movable half of the die 8 with its refractory metal insert 10 away from the fixed half 9 and its insert 11, such as by means of toggle linkages l2 and I3. Suitable die casting machines known in the art provide substantial restraint between die backup block halves 8 and 9, and the dies are restrained in relation to the energy source 7 for plunger 4, so that the dies will not be forced open by the very large pressures generated in the liquid metal casting by plunger 4!. Separating forces tending to force the dies open can be quite large, depending on the pressure used and the projected area of the casting. In the present invention, injection pressures preferably in the order of about [,000 to 10,000 pounds per square inch are generally used, although substantially lower or higher pressures may be used within the scope of the invention.

FIG. 2 illustrates the die casting machine of FIG. I in which plunger 4 has forced liquid metal 5 into the casting cavity 6. Excess molten metal is present in the feeding system 14 which includes biscuit l8, and gate 16. In accordance with one aspect of the invention, the liquid metal 5 is preferably caused to move into the cavity 6 very rapidly, depending on size such as in considerably less than I second, and would normally freeze substantially completely within less than I or 2 seconds after it fills the mold. As can be seen in H6. 2, the plunger 4 seals off the metal supply opening 15 as plunger 4 advances past opening 15, so that molten metal 5 is forced into die cavity 6.

As stated elsewhere in this application, the refractory metal inserts are maintained at temperatures high enough to prevent premature freezing of the cast metal to avoid casting surface defects and faithfully reproduce the die cavity surfaces, generally above about 500 F., the actual temperature depending on the metal being cast. Such temperatures can be attained by the use of electric heaters 19 in the die block itself, by preheating the die blocks internally or externally with torches or otherwise before commencing casting operations, or by heat from the metal being cast. With one set of dies used by applicants, 4 kilowatts of electric heat input was more than sufficient to raise the dies to and maintain them at about 700 F. operating temperature. Once casting operations are commenced, the casting operations themselves tend to keep the dies at quite high elevated temperatures, and supplementary heat input may or may not be necessary, depending on the temperature of the metal, the time of the casting cycle, the heat absorbing ability of the die casting machine and its environment, and other factors.

FIG. 3 illustrates the same die casting machine after the casting has solidified. Toggles l2 and 13 have opened to pullthe movable die half 8 with its refractory metal insert 10 away from fixed die half 9 with its refractory metal insert 11. Simple means such as knock-out pins known in the art are normally provided to remove casting 17 with its solidified biscuit 18 from the fixed half 9 of the die backup block once the movable half 8 has moved out of the way. After removal of casting 17, the biscuit l8 and gate 16 can be cut off at section X-X. Plunger 4 has retracted beyond the metal inlet 15 to allow metal to be poured in for the next casting. The dies then can be closed again as illustrated in FIG. 1 to prepare for the next casting cycle.

FIG. 3a illustrates another embodiment of a die made for use with the invention having a relatively thin layer of refractory metal liner 10a in the die instead of the more massive inserts l and 11 ofFlGS. l, 2 and 3.

The efficient thermal coupling of the refractory metal insert or liner to the heat sink, which can be the die backup blocks 8 and 9 or cooling water, or other means, can be accomplished by carefully matching and fitting the insert or liner into the backup blocks. Of course, this is morecritical for thin liners than for massive inserts. Thin liners could be physically bonded to the backup blocks as by brazing with a material that is a good heat conductor.

Although sometimes preferable, it is not necessary that refractory metal inserts as massive as those illustrated at 10 and 11 be used. The necessary minimum thickness of the refractory metal layer will depend on the backup material, as well as the nature of the ferrous alloy to be cast. With a backup material that has high heat conductivity, thinner refractory metal layers can be used, depending also on the respective thermal expansion characteristics of the backup and liner materials. Also, composite dies made of several layers of different materials are conceivable, so long as the material facing the molten metal is a refractory metal of the invention and is thick enough to control the heat transfer characteristics of the mold and withstand the rigors of repetitive casting. Preferably, massive inserts of at least 15-inch minimum thickness of refractory metals are used in steel molds. However, thinner inserts which might be made by plasma spraying or otherwise bonding suitable refractory metals as cladding on other materials such as copper alloys or die steels could have a refractory metal thickness as little as about 0.06 inch or less.

A suitable refractory metal is unalloyed molybdenum produced by conventional commercial powder metallurgical techniques. unalloyed tungsten, also preferably produced by powder metallurgical techniques, or produced by are melting or electron beam melting, also may be desirable. For increased strengths at higher temperatures, generally with some sacrifice in heat transfer characteristics, alloys such as, for example, molybdenum strengthened by the addition of small amounts of precipitate-phase forming elements such as titanium, zirconium, and hafnium, together with carbon, nitrogen, boron or other elements, or tungsten strengthened by the addition of rhenium or other soluble alloying additions, are use in] as molds and mold lining materials. Other molybdenum and tungsten metals, wrought tungsten, wrought molybdenum, and alloys or composites that have suitable thermal characteristics can be used, including tungsten containing a dispersed phase of about 2 percent thon'a, tungsten or molybdenum powders liquid-phase sintered with nickel, iron or other metals, and, in some applications, copperor silver-infiltrated porous pressed and sintered tungsten. Of course, if the refractory metal part of the die is thick enough, it can be used without die steel or other backup material.

The extreme turbulence and rapid cooling effect of pressure injection into refractory metal molds, together with post-casting thermal transformations facilitated by such effects, largely determine the structure of ferrous metal castings made in accordance with the present invention. These effects control the entire structure of thin castings, and they cause quite thick outer layers of fine-grained structure in thicker castings, with the thickness of the outer layer depending on the metal. It can be as thick as one-half inch or more in gray iron, thinner in highly alloyed irons which have poorer heat conductivity. To

' take full advantage of the invention, the working portions or highly stressed portions of castings should be thin enough to have uniform structures and grain sizes characteristic of the invention throughout their cross sections. Thus, the cross sections in these portions should not be so thick that the preferred structure does not extend substantially through it. Other portions of the same casting can be made thicker and have a cored structure with more conventional structure and grain sizes at the center, such as useful die cast objects of malleable iron having centers or cores of gray iron.

To meet the requirements of the invention, the mold or die, considered as a structural entity, must have certain minimum properties of strength, thermal absorptivity and conductivity, and melting point. To resist plastic deformation at the maximum temperatures and pressures reached in pressure injection die casting of ferrous alloys, the yield strength of the die material, whether in a monolithic block or within a composite or layered structure, such as for example a layer of molybdenum on a backing of alloy steel or copper-beryllium alloy, should be such at every point within the structure so as to resist the stresses at that point as determined by injection pres sure, cavity geometry, and distance of that point from the mold-casting interface. Furthermore, it must be capable of resisting the stresses at the maximum temperatures reached at that point during repetitive casting of high melting ferrous alloys. Further, the thermal absorptivity, which is the heat diffusivity, of the mold structure is such that the heat of fusion together with any usual degree of superheat can be removed from the critical section of the casting with sufficient rapidity to achieve the desired fine-grained cast structure. Also, it is desirable that the thermal conductivity of the mold structure be sufficient so that such heat of fusion, superheat, and any portion of residual heat transferred to the mold following solidification but prior to ejection of the casting, be transferred efficiently by the refractory metal mold liner or insert to the heat sink, such as into cooling water, conducted to the body of the casting machine, or radiated to the atmosphere, or otherwise disposed of, with sufficient rapidity so that the average temperature of the mold surface preferably remains essentially constant with time during extended sequences of repetitive casting, or at least does not reach deleteriously high levels. The average operating temperature is determined from the maximum and minimum temperatures occurring during cyclical operation. Further, it is highly desirable that the mold surface have a melting point substantially higher than that of the metal being cast. This aids in resisting wetting and erosion by the liquid metal and, most importantly, allows the mold to be heated to substantially elevated temperatures prior to casting so as to avoid premature surface freezing of the casting, yet without endangering the finish and integrity of the mold surface as a result of the higher instantaneous surface temperatures experienced during contact with injected liquid metal.

In brief, to meet the requirements of this invention, the properties of the mold must be such that it not only has a high degree of permanence for extended casting operations, but

also such that the precise dimensions and surface finish of the casting produced therein are detennined by the heated inner surfaces of the mold itself rather than by a prematurely frozen, often imperfect, skin of metal instantaneously frozen at cold mold surfaces.

It will be apparent that the minimum properties which a mold must have to meet the above requirements will depend on several parameters including: the size and shape of the casting, the precasting mold temperature, the temperature at which the liquid metal is injected into the mold, the pressure of injection, the heat capacity, density, and thermal conductivity of the particular metal or alloy being cast, and other factors. It also will be apparent that different combinations of refractory metal mold surface and substrate can be devised to meet such requirements, depending on all the above parameters as well as the particular mold material, backup material, heat-sink mechanism, and other parameters.

The invention is applicable to the production of a broad variety of articles of commerce cast from ferrous metals containing at least 50 percent iron. By way of brief example, and not limiting the scope of the invention, the invention can be used to produce such products as: automotive components such as rocker arms, steering knuckles, bearings, and fittings; appliance parts such as linkages, gears, valves, and pulleys; architectural fittings; miscellaneous hardware; and many other types of products.

Iron-Carbon Diagram FIG. 4 is the iron-rich end of the iron-carbon phase diagram showing the phases present at metastable equilibrium at the indicated temperatures with the indicated percentages of carbon in a binary iron-carbon alloy. The metastable nature of the equilibrium stems from the fact that Fe C, called iron carbide or cementite, is thermodynamically unstable at elevated temperatures with respect to decomposition to free carbon and iron-carbon solid solution. Upon addition of other alloying elements such as chromium, nickel, phosphorus, and silicon, various shifts in the diagram will occur. To describe the diagram in general terms, pure iron containing no carbon is seen to melt at about 2,800 F. The minimum melting point for the eutectic composition occurs with 4.3 percent carbon at 2,066 F. With other alloying additions, commercial gray cast irons can be found to melt at lower temperatures.

Ferrite (a-iron) and B-iron have body-centeredcubic (BCC) crystalline structures, while austenite (y-iron) has a face-centered-cubic (FCC) structure. Austenite, ferrite and S-iron are solutions of carbon in iron. Austenite normally is not stable at temperatures below about 1,333" F., but its stability at lower temperatures may be enhanced by certain alloying additions, such as nickel, for example.

Iron-carbon alloys containing more than about 1.7 percent carbon or the equivalent thereof, the maximum amount of carbon that is soluble in austenite, are known as cast or graphitic irons, and are characterized by the presence of free carbon or graphite as a dispersed phase after heat treatments of the iron. The matrix phase may retain more or less carbon in solution depending on the nature and duration of the heat treatment.

The morphology of the various phases in iron-carbon alloys is most important in determining the strength and other properties of ferrous alloys. Iron-carbon alloys are quite sensitive to heat treatments which cause variations in their structure and can harden or soften the metal in various ways.

The cast irons often contain silicon and other alloying elements. The carbon equivalent of a cast iron is detennined by adding to the actual percentage carbon content one-third of the silicon percentage and making other adjustments known in the art for other elements present such as phosphorus. With high carbon equivalent levels, such as over about 4 percent, the solidified product normally is gray cast iron, which is a matrix of ferrite with dispersed platelets of graphite. If any pearlite is present, suitable heat treatment can convert it to ferrite and graphite. For many gray irons, if higher strength is desired, the casting can be annealed to remove casting stresses at lower temperatures, leaving the gray iron matrix in a pearlitic condition. Carbon equivalents over 4.3 percent are generally undesirable since coarse graphite flakes, known as kish, form in the melt and can deleteriously affect the castings. Graphite flakes in cast iron can be considered mechanically as almost a notch or void, so the size, shape and distribution of graphite in cast irons is most important. Thus, the fine grain size and small graphite particles in products of the invention are very desirable.

Because of the greatly lowered melting point, cast irons are less costly than steels and preferable to steels for many applications where they can be used. Also, solidification shrinkage of the castings on cooling is at a minimum for most gray cast irons, thus facilitating the casting of complex parts. However, gray cast irons are notoriously weaker and more brittle than steels because of the graphite in the structure.

To obtain greater ductility in cast irons, malleable iron or ductile iron can be used. Malleable iron is produced by annealing white cast iron (a metastablestructure containing no free carbon) to cause the graphite to form as relatively compact nodules or irregular spheroids, similar in shape to popcorn balls, in contrast to the elongated flake graphite types in gray cast iron. The white cast iron from which malleable iron is produced normally is a low carbon cast iron, such as containing 2.5 percent carbon and 1.5 percent silicon for a carbon equivalent of 3.0 percent; White iron can be produced with lower carbon contents in sand molds and with relatively high carbon contents in chill molds. White iron is cast iron containing ferrite and cementite, generally with more or less pearlite, and no free graphite. lt is quite strong, hard and brittle. On proper annealing, the cementite decomposes to give graphite nodules in ferrite, or, if preferred, in a pearlitic matrix. This is malleable iron and is much more ductile and much softer than white iron, and tougher than gray iron.

Graphitizing agents such as silicon, nickel and copper encourage the formation of gray iron rather than white iron. Duplex malleable/gray iron metal articles of the invention ascast have a white iron encasement or surface around a gray iron core. This white iron can be converted to a malleable iron by appropriate heat treatment, and graphitizing agents can be used to minimize thickness of the white iron case. Suitable heat treatments can convert the white iron at the surface to a malleable iron and leave the gray iron core in the desired metallurgical condition.

This emphasizes a particular advantage of this invention in that, for example, a wide range of gray iron compositions can be cast and converted to fine-grained iron with malleable properties, even though the content of carbon considerably exceeds that of the normal malleable irons. Also, the duplex structure comprising malleable iron with a gray iron core has certain properties improved over those of gray iron, such as strength and ductility. It also has the advantage of lower cost as compared with malleable iron itself, due partly to lower melting points and partly to shorter time or lower temperature heat treatments.

Ductile iron with regular spherical graphite particles can be produced by adding certain inoculants, such as magnesium, to molten graphitic iron compositions, just before pouring the castings. This type of structure can be further refined and developed by post-casting thermal treatments.

Introduction to Specific Examples Although the beneficial effects of the invention still cannot be explained fully, even in hindsight, the following thoughts and hypotheses will aid in understanding to a certain extent some mechanisms which may contribute to the greatly improved properties of articles of commerce made according to the invention, which properties are more fully disclosed in the specific examples.

From an examination of the metallurgical structures of ferrous articles pressure injection die cast in refractory metal molds, such as those structures depicted in FIGS. a, 6a, and 8a, it will be apparent to those skilled in the metallurgical arts that the structures of said metallic articles are characterized by an unusual degree of fineness of grain, in some cases more than an order of magnitude smaller in average unit grain area than normally is observed for metal of similar composition cast by conventional techniques, such as by sand mold casting as depicted in FIGS. 5b, 7b, and 8b. Smooth-surfaced, ex-

tended bodies of ferrous alloy metals having such uniform,

. generally nondendritic, exceedingly fine-grained, as-cast structures have not been generally commercially available heretofore, owing partly to the great difiiculty in achieving rapid removal of the heat of fusion and superheat from a liquid ferrous metal through the surface of a conventional mold material, yet with the avoidance of premature chilling and freezing which would lead to surface defects and casting imperfections. Further, in metal sections up to at least one-half inch in thickness, a remarkable uniformity of such exceedingly fine cast grain structures across the entire thickness of the section occurs in pressure die cast ferrous alloys, such as low carbon and alloy steels for example. Also, the usual type of nucleation and grain growth progressing slowly inward from the surface, which leads to an undesirable dendritic type of cast grain structure, appears not to be operative in the solidification of such cast sections excepting only with certain ferrous alloys which have low thermal conductivity, such as certain stainless steel compositions.

it is difiicult to explain this unusual uniformity of exceedingly fine grain structure on the basis of hitherto known metallurgical practices. We believe that the very rapid injection of the mass of molten metal through the narrow gating system into a preheated cavity mold of high thermal conductive and absorptive properties may allow a supercooling of the entire mass of the casting to a remarkable degree immediately prior to solidification, yet without premature freezing at mold wall surfaces, the degree of supercooling being further accentuated by the application of pressure during the freezing process, so that the supercooled molten metal nucleates and solidifies very quickly and nearly uniformly throughout its cross section rather than the usual process of solidification common to most casting practices in which the metal slowly and sequentially freezes from the mold wall interface into the center of the casting. The normal freezing process leads, of course, to differences, sometimes drastic, in grain structure from edge to center in ferrous castings, to undesirable segregation of impurities as well as intentionally added alloying agents from point to point within ferrous castings, and to inherently weak, dendritic grain structures. These difficulties are particularly accentuated in liquid metal pressing or squeeze casting.

Owing to the uniform and exceedingly fine-grained structures of castings of the invention, desirable alterations or transformations in metallurgical structure to be produced by thermal treatments, such as the conversion of cast white iron to a nodular graphite dispersion in a ductile ferrite matrix to produce malleable iron, can be accomplished in much shorter times and at lower temperatures than are conventionally employed in the ferrous metallurgical art and with retention of a desirable, uniform fine grain size as illustrated in FIGS. 7a and 8c, and with improvements in mechanical properties, as discussed below. The shorter and/or lower temperature heat treatments are, of course, of significant economic advantage. We believe such phenomena may be due in part to the relatively short diffusion distances necessary for structural transformations to occur in exceedingly fine-grained structures, together with the somewhat higher interfacial free energies of such structures. Further, the combination of ultrafine grain size together with the repression of segregation of impurities at grain boundaries during the rapid solidification process appears to significantly improve the resistance of such materials to corrosion and attack by chemicals and oxidizing agents.

SPECIFIC EXAMPLES Example I Duplex Malleable/Gray Cast iron A ferrous metal alloy of the composition: 3.5% carbon, 2.2% silicon, 0.7% manganese together with incidental amounts of other elements such as phosphorus, sulfur and others, balance iron, a member of the class of alloys known generally as gray cast iron and designated specifically as having a carbon equivalent of 4.23 percent, located as Ex. I on FIG. 4, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2,400 F. into the injection chamber of a pressure injection die casting machine, whereupon it immediately was caused to flow under the action of a plunger moving at a rate of 35 feet per minute (ft/min.) and injected under a pressure of 3,000 pounds per square inch (p.s.i.) through a narrow gate approximately 0.125X0.500 inches into a shaped cavity previously formed within adjoining die insert blocks of pressed and sintered molybdenum fitted into a backup steel mold block, said process being as illustrated in FIGS. 1 and 2. Prior to injection of the molten ferrous alloy, the mold surfaces were maintained at a temperature of about 500 F., in part by internal, controllable, electric heaters built within the mold structure behind the mold surfaces and in part by residual heat from previous castings made in the same mold. Within a period of less than about 2 seconds, the casting had substantially solidified, reproducing precisely the shape and closely duplicating the surface of the pressed and sintered molybdenum mold cavity, whereupon the adjoining halves of the mold cavity were opened by the mechanism of the casting machine and the solid metal part was ejected as illustrated by FIG. 3. By substantially solidified is meant that the casting was solidified sufiiciently to allow its removal from the mold. It is not known whether some molten metal might then still be present at the center of the casting. The casting comprised an outer layer of white iron with an inner core of gray iron. The unusual as-cast grain structure of the center of the solidified casting is illustrated in FIG. 5a. Heat treating at L650 F. for about 2 hours converted the working surfaces of the casting to malleable iron. Properties of this cast metal are documented in table II in comparison with properties for metal of the same composition cast in sand molds and given the same anneal.

TABLE I1 in the tables, ksi means thousands of pounds per square inch, and R means hardness on the Rockwell 8" scale. Rupture tests were made by three-point bending of bars 3X1 Axle inches with 2 inches between supports. All tests were made at room temperature of about 77 F. Tensile tests were mostly performed on standard machined button head specimens with a gauge diameter of 0.250 inch and length of about 1.3 inches, using an elastic strain rate of 0.005 inchlinch/minute (in./in./min.) and a plastic strain rate of 0.05 in.lin./min.

The annealing treatment results in a maximum soft condition so as to allow equal comparison of properties without regard to strength improvements that may be obtained by other known heat treatments.

The finer graphite flake size and more uniform distribution of the flakes in the gray iron core, as compared to the sand cast article (FIG. 5b) of equivalent chemical composition, have contributed to higher tensile strengths and higher rupture stress values for the pressure injection die cast article.

Example 2 Malleable Cast Iron A ferrous metal alloy of the composition: 2.5% carbon, l.5% silicon, 0.45% manganese, 0.6% molybdenum, together with incidental amounts of other elements such as phosphorus, sulfur and others, balance iron, to be cast as white iron, having a carbon equivalent of 3.0 percent, located as Ex. 2 on FIG. 4, and subsequently to be heat treated to form malleable cast iron, was melted in an electric induction furnace. A suitable amount of this metal in the molten condition was ladled at a temperature of about 2,450 F. into the injection chamber of a pressure injection die casting machine, whereupon it was cast in the same manner as example 1. The as-cast grain structure of the solidified white cast iron metal part is illustrated in FIG. 6a, and the corresponding malleable iron structure after a thermal annealing treatment of 2 hours at 1,650 F. are illustrated in FIG. 7a. Mechanical properties of the heat treated or annealed cast metal are documented in table II] in comparison with typical properties for metal otherwise the same but cast in sand molds and then malleablized or annealed for times in excess of at least 48 to 60 hours at a temperature in the range of l,600 to 1,700 F., as is customary in the art of making malleable iron.

TABLE III Malleable lron With the exception of the fineness of the cells, only minor differences can be detected between die cast and sand cast white cast iron structures. Of significant economic importance however is the ease with which the white cast iron can be converted to a malleable iron. Heat treatments at least as short as one-tenth as long as usual commercial practices can be used to obtain the structures shown in FIG. 7a. The fineness of the die cast malleable iron structure shown in FIG. 7a as compared to a sand cast malleable iron structure, as shown in FIG. 7b, also contributes to higher strengths and hardnesses. Optimization of chemistry and treatments would lead to even further improved properties.

Example 3 Ductile Cast Iron Copper-free pig iron having the following composition was used as a charge material: 4.4% carbon, 010% manganese, 0.029% phosphorous, 0.028% sulfur, and 0.73% silicon. In order to lower the carbon content, percent Armco iron was added to the charge. Another 1.4 percent silicon metal was added to increase the silicon content to the desired level.

Induction melting of a l00-pound charge was accomplished in a ZOO-pound capacity furnace. Temperature of the melt was maintained at 2800-2850" F. before inoculation. Carbon and silicon were quickly determined by the Leco and X-ray spectrographic methods respectively. Carbon was found to be 3.6 percent and silicon 2.1 percent.

Two methods have been employed to make the addition of about 0.12 percent magnesium in die casting. For ladle inoculation, the ferrosilicon-S percent magnesium inoculant was placed in the transfer ladle and the molten metal was poured on top of the inoculant. For furnace inoculation, the required amount of magnesium-nickel alloy inoculant was immersed in the induction furnace, after which the melt was ladled for die casting. In order to assure good nodule formation, a ferrosilicon post inoculant was employed here.

Describing the first mentioned of inoculation, 10 to grams of ferrosilicon-magnesium plus 5 to 7 grams of silicon were used in a ladle used for transfer of 2.2 pounds of molten metal to the die casting machine. The transfer ladle was made of cast iron, coated with a protective glass called Arco Perm 100. A sequence of die casting was as follows: place the inoculant in the ladle, pour 2.2 pounds of metal on top of inoculant, wait a few seconds for the reaction to subside, finally pour into the shot sleeve and commence die casting. Half a dozen castings were made by each inoculation method to insure reproducibility.

The thickness of the die cast parts ranged from three-sixteenth of an inch to twenty-one thirty-seconds of an inch. Because of the chilling effect in die casting, the total thickness of the parts appeared, upon fracture, to be white, and the microstructure is shown in FIG. 8a. After annealing at 1700 F. for 30 minutes, the castings transformed to nodular iron, as illustrated in FIG. 8c. Mechanical testing of a 1 -inch gauge length sample at a strain rate of 0.02 in./in./min. of die cast nodular iron annealed for two different times showed the fol- Without any anneal, the sand-cast nodular iron had a Brinnell Hardness Number of about I85, indicating that the die cast nodular iron, even after the wk-hour anneal, is considerably harder and stronger than the sand-cast metal.

Die casting of nodular iron has now been successfully demonstrated. Small diameter nodules and a fine grain size are obtained in die casting nodular iron. Such fineness results in superior mechanical properties such as high strength, hardness, ductility and toughness. A wide range of mechanical properties is foreseen in die cast nodular iron parts due to the small size nodules obtained.

Example 4 Duplex Malleable/Gray Cast Iron Duplex malleable/gray cast iron was cast and then converted by heat treatment successfully as in example I, but in wrought molybdenum die inserts using a plunger speed of I30 ft./min. and a pressure of 3000 p.s.i.

Example 5 Malleable Cast Iron Malleable cast iron was produced successfully as in example 2, but in copper-infiltrated tungsten die inserts using a plunger speed of I30 ft./min. and a pressure of 9000 p.s.i. Examples 2 and 5 show that white cast iron can be produced by use of the invention with unusually high carbon equivalents. Broader ranges of composition can be used with the present invention generally, and particularly to produce malleable iron, than with methods of the prior art.

The foregoing is a description of illustrative embodiments of the invention, and it is applicants intention in the appended claims to cover all forms which fall within the scope of the invention.

We claim:

1. A process for repetitive pressure injection die casting and heat treating of articles of graphited iron compositions containing at least 50 percent by weight of iron and more than the maximum amount of carbon that is soluble in the composition, comprising the sequential steps of;

A. rapidly injecting said ferrous metal while molten into a closed die through a gate so that the molten metal nucleates and solidifies nearly uniformly in said die, at least the interior surfaces of said die being a refractory metal selected from the group consisting of molybdenum, tungsten, and alloys containing at least 50 percent by weight of one or more of molybdenum and tungsten, said refractory metal being efliciently thermally coupled to a heat sink to permit rapid extraction of heat from said ferrous metal, said refractory metal, at its average operating temperature, having a thermal diffusivity of at least about I ft.lhr., a heat diffusivity of at least about 40 B.t.u./ft.'

F. hr. and a melting point above about 3,000 F., said refractory metal being thick enough to enable control of the heat transfer characteristics of the mold and adequate to withstand the rigors of repetitive casting, and said process being operated with the surfaces of said dies at a sufficiently elevated average operating temperature above about 500 F. to substantially prevent surface irregularities in the cast articles due to premature freezing, but substantially below the freezing point of said ferrous metals,

B. holding said ferrous metal under pressure in said closed die while heat is extracted rapidly from said ferrous metal through said refractory metal until said article is substantially solidified.

C. removing said article from said die, and

D. heat treating said article to modify its metallurgical structure by precipitating carbon from solution as graphite, said precipitated graphite being essentially all in the form of dispersed nodules, in less than about 4 hours, and in the temperature range of about l500 to 2000 F. to modify its metallurgical structure by precipitating carbon from solution as graphite essentially in the form of dispersed nodules, and to improve mechanical properties of said article.

2. A process according to claim 1 in which the surfaces of said dies are maintained at an average operating temperature in the range of about 700l000 F.

3. A process according to claim 2 in which said refractory metal is selected from the group consisting of molybdenum and alloys containing at least 50 percent by weight of molybdenum.

4. A process according to claim 2 in which said refractory metal is selected from the group consisting of tungsten and alloys containing at least 50 percent by weight of tungsten.

5. A process according to claim 2 in which said article as cast has the metallurgical structure of white iron with a core of gray iron, and

after said heat treating, the metallurgical structure of said article is essentially that of fine-grained malleable iron having a core of fine-grained gray iron.

6. A process according to claim 5 in which said heat treating comprises heating said article at a temperature in the range of about l500 F. to about I900 F. for a time in the range of about 10 minutes to about 2 hours, to convert said white iron to malleable iron.

7. A process according to claim 2 in which said article when cast has essentially throughout the metallurgical structure of white iron, and

after said heat treating, the metallurgical structure of said article is essentially that of malleable iron.

8. A process according to claim 7 in which said heat treating comprises heating said article at a temperature in the range of about l500 F. to about 1900 F. for a time in the range of about 10 minutes to about 4 hours, to convert said white iron to malleable iron.

9. A process according to claim 2 in which while said ferrous metal is molten, an inoculant is added to it which causes spheroidization of graphite as it forms during cooling and subsequent heat treatment of the cast article, and

after said heat treating, in the metallurgical structure of said article essentially all of the precipitated graphite is in the form of spheroidal modules.

10. A process according to claim 9 in which said heat treating comprises heating said article at a temperature in the range of about 1500" F. to about 2000 F. for a time in the range of about 10 minutes to about 2 hours to increae the amount of graphite precipitated in the form of spheroidal nodules.

11. A product of the process of claim 1.

12. A product of the process of claim 5.

13. A product of the process of claim 6.

l4. A product of the process of claim 7.

15. A product of the process of claim 10. 16. A process of claim 1 in which the ferrous casting solidifies at a rapid enough rate to permit ejection of said casting from said die in a time of less than about 2 seconds for castings having a maximum thickness of no more than about one-half inch.

i 1 i i t 

3. A process according to claim 2 in which said refractory metal is selected from the group consisting of molybdenum and alloys containing at least 50 percent by weight of molybdenum.
 4. A process according to claim 2 in which said refractory metal is selected from the group consisting of tungsten and alloys containing at least 50 percent by weight of tungsten.
 5. A process according to claim 2 in which said article as cast has the metallurgical structure of white iron with a core of gray iron, and after said heat treating, the metallurgical structure of said article is essentially that of fine-grained malleable iron having a core of fine-grained gray iron.
 6. A process according to claim 5 in which said heat treating comprises heating said article at a temperature in the range of about 1500* F. to about 1900* F. for a time in the range of about 10 minutes to about 2 hours, to convert said white iron to malleable iron.
 7. A process according to claim 2 in which said article when cast has essentially throughout the metallurgical structure of white iron, and after said heat treating, the metallurgical structure of said article is essentially that of malleable iron.
 8. A process according to claim 7 in which said heat treating comprises heating said article at a temperature in the range of about 1500* F. to about 1900* F. for a time in the range of about 10 minutes to about 4 hours, to convert said white iron to malleable iron.
 9. A process according to claim 2 in which while said ferrous metal is molten, an inoculant is added to it which causes spheroidization of graphite as it forms during cooling and subsequent heat treatment of the cast article, and after said heat treating, in the metallurgical structure of said article essentially all of the precipitated graphite is in the form of spheroidal nodules.
 10. A process according to claim 9 in which said heat treating comprises heating said article at a temperature in the range of about 1500* F. to about 2000* F. for a time in the range of about 10 minutes to about 2 hours to increase the amount of graphite precipitated in the form of spheroidal nodules.
 11. A product of the process of claim
 1. 12. A product of the process of claim
 5. 13. A product of the process of claim
 6. 14. A product of the process of claim
 7. 15. A product of the process of claim
 10. 16. A process of claim 1 in which The ferrous casting solidifies at a rapid enough rate to permit ejection of said casting from said die in a time of less than about 2 seconds for castings having a maximum thickness of no more than about one-half inch. 